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Glass Knifefish
Eigenmannia virescens
A ghostly, completely translucent creature. It uses weak electric radar to navigate, but unlike the Black Ghost, it can 'tune' its frequency to peacefully school with its own kind. Extremely shy and demands a heavily planted, dark tank with fine sand to thrive.
Water Temperature
22 - 28 °C
pH Value
6 - 7
Water type
Freshwater
Tank level
Bottom
Adult size
35 cm
Minimum tank volume
>= 300 L
Minimum group
>= 1
GH
2 - 12 dGH
KH
1 - 6 dKH
TDS
40 - 240 ppm
Conductivity
63 - 375 uS/cm
Species description
Geographical Origin and Habitat: Amazon and Orinoco River basins, in South America. Lives among the roots of flooded forests and dense patches of submerged vegetation in murky waters.
Taxonomy and Morphology: Glass Knifefish / Green Knifefish (Eigenmannia virescens). Neotropical "knife" fish. Transparent/diaphanous and lacking dorsal and caudal fins, it swims exploiting the undulations of the very long anal fin in an almost hypnotic motion (backwards and forwards in the same way).
Social Behavior: Crepuscular and nocturnal. Extremely gregarious: lives in schools, using weak bio-electric fields (electrogenesis) to communicate and orient itself (EOD). Isolated, it wastes away.
Coloration and Sexual Dimorphism: Translucent/glassy body with greenish reflections. The internal organs can be glimpsed. Males usually have stronger EODs and larger sizes, but are indistinguishable by sight.
Care, breeding and tankmates
Aquarium Setup: Long tank (at least 120-150 cm / 48-60 inches). Decor: "Chaos". Woods of tangled timber, PVC pipes or thick bushes of plants, dim lighting. Requires space so as not to electrically trip over conspecifics, even while crowding together by day.
Diet and Feeding: Insectivore and carnivore. Must be fed strictly with the lights out. Prefers bloodworms, tubifex (washed) and small live/frozen invertebrates. Will ignore pellets and flakes for a long time.
Water Quality: Seeks clean and well-filtered water; powerful filtration must not, however, translate into a "torrent" style water jet. Oversized biological filtration is preferable (peat waters).
Compatibility and Cohabitation: Ideal with peaceful South American fish that sleep at night, such as large schools of Tetras and Corydoras, provided they are not so small as to fit in its mouth. No aggressive cichlids (Astronotus).
Aquarium Breeding: Very rare in captivity, it is induced by the simulation of the rainy season (temperature drops, spray rain and great availability of live food). The eggs are laid in the fronds (mosses).
Risks and Diseases: Fish lacking classic scales, it is Hypersensitive to chemical cures, copper and malachite green (halved doses mandatory). Do not cut or entangle the long "tail" which acts as a sensory radar.
Geographic Origin and Distribution
Amazon and Orinoco River basins, in South America. Lives among the roots of flooded forests and dense patches of submerged vegetation in murky waters.
Origin
Amazon, Orinoco, and Guianas
Fish profile
- Temperament
- Semi-aggressive
- The Glass Knifefish is generally timid and peaceful, preferring to hide away from bright lights and boisterous tank mates. They are fascinating to watch due to their unique method of locomotion, rippling their elongated anal fin to move forward and backward effortlessly. While peaceful to similarly sized fish, their predatory nature towards very small fish must be respected. They are intelligent and can learn to recognize their owner during feeding time.
- Diet
- Omnivore
- Eigenmannia virescens is a strict carnivore that relies on live and frozen foods. Their diet should consist mainly of bloodworms, blackworms, brine shrimp, and chopped earthworms. They rarely accept dry foods like flakes or pellets, though some may be trained to eat sinking carnivore pellets over time. Feeding them after the aquarium lights have turned off ensures they get their share of the food.
- Life expectancy
- 6-12 years
- The glass knifefish can live for 6 to 12 years in captivity when provided with proper care. Achieving their maximum lifespan requires maintaining stable water parameters and providing a stress-free environment. They are scaleless fish and are highly sensitive to chemical treatments, so avoiding copper-based medications is essential for their long-term health.
- Adult size
- 35 cm
- The catalog's scalar value is 35 cm. The FishBase SPECIES table (v25.04 snapshot) source reports a maximum adult measurement of 44 cm for this species, recorded as standard length (SL). The taxonomic record consulted is Eigenmannia virescens (v25.04); male or unsexed record. These figures are not interchangeable: the first is the profile's editorial reference, whereas the second is a published maximum under a specific measurement convention; differences among sources can reflect sex, population, life stage and measuring method.
- Sex ratio
- Solitary
- There are no obvious physical differences between males and females, making sexing them nearly impossible for the average aquarist. They are best kept either singly or in a group of five or more to disperse any conspecific aggression. Keeping just two or three can result in the dominant individual bullying the weaker ones. In a large enough tank with plenty of cover, a group can coexist peacefully.
- Feeding frequency
- Twice daily
- Being predominantly nocturnal, they should ideally be fed once or twice a day, with the main feeding occurring right before or after lights out. They use their electrical organ to locate prey in the dark, making them highly effective nighttime hunters. Ensure that daytime feeders do not consume all the food before the knifefish has a chance to eat. Consistent evening feedings will keep them healthy and well-nourished.
- Bioload
- High
- The Glass Knifefish produces a moderate bioload compared to other fish of similar size. They are carnivorous and require meaty foods, which can degrade water quality quickly if overfed. Strong biological filtration and regular, substantial water changes are necessary to maintain pristine conditions. High water quality is critical, as they are scaleless fish and very sensitive to ammonia and nitrite spikes.
- Flow
- Stagnant
- They prefer a gentle to moderate water flow that mimics the slow-moving rivers and streams of their native South American habitat. Strong, turbulent currents will stress them and make it difficult for them to navigate using their weak electric fields. The flow should be just enough to ensure good filtration and oxygenation without creating a whirlpool effect. Planted tanks with natural flow breaks are ideal.
- Reproduction
- Oviparous
- Breeding the Glass Knifefish in a home aquarium is extremely rare and difficult to achieve. They breed during the rainy season in the wild, requiring significant changes in water level, temperature, and conductivity to trigger spawning. The females lay eggs among floating plants, and the parents offer no care to the fry. Specialized breeding setups simulating seasonal Amazonian floods are required for any chance of success.
- Compatibility & tankmates
- They are generally peaceful towards fish that are too large to fit in their mouths. However, they are nocturnal predators and will readily consume small tetras, rasboras, or shrimp during the night. Good tank mates include peaceful cichlids, larger tetras, and corydoras. They can sometimes be territorial towards other knifefish, so hiding spots are essential if keeping a group.
- Tank level
- Bottom
- During the day, they will hide in the lower levels of the tank among plants, driftwood, and PVC pipes. They are shy and require these hiding spots to feel secure and minimize stress. At night, they become highly active and will utilize the entire water column as they forage for food. Providing a dimly lit tank or a dense canopy of floating plants can encourage them to venture out during the day.
Scientific Parameters & Models
FishBase
Window to the official scientific data of the species
Data synced on: 06/15/2026
- Trophic level
- 3.1 ± 0.3 se
- Resilience
- Medium (1.4 - 4.4 years)
- Phylogenetic uniqueness (PD50)
- 0.5
- Bayesian Length-Weight
- a = 0.00229, b = 2.9
- VulnerabilityModerate vulnerability (41 / 100)
Image gallery
Licensed images linked to the species or, when marked, to the closest representative taxon.
Related species
Recommendations grouped by close relatives, companion choices, and similar care.
